The production of ferroboron has generally been carried out by aluminothermic reaction, the oxidic boron containing new material and iron oxide being reduced with the aluminum and smelted. The product is usually an aluminum-containing ferroboron which consists of say 15 to 18% by weight boron, up to about 4% by weight aluminum, a maximum of 1.0% by weight silicon, a maximum of 0.10% by weight carbon, the balance being iron and the usual impurities or trace elements associated with iron.
Alternatively, the aluminothermic method can produce a product which contains 18 to 20% by weight boron, up to 2% by weight aluminum, a maximum of 2% by weight silicon, a maximum of 0.10% by weight carbon, the balance being iron and unavoidable elements which do not materially affect the properties of the product.
These ferroboron compositions often are desirable for use in the production of metallic glasses and for this purpose the presence of aluminum is detrimental since the aluminum is easily oxidized and the resulting oxides interfere with the formation of the metallic glasses. Similar disadvantages reside in the use of aluminothermic and like methods for the formation of ferroborosilicon.
Consequently, when the alloys are to be used in the production of metallic glasses, it is common to use a carbothermic reduction of the oxidic boron and iron carriers, thereby producing low aluminum ferroboron alloys or ferroborosilicon alloys.
The procedure can be carried out in an electrical furnace using a charge or burden which employs as its carbon carrier the finely divided materials such as milled coal or milled coke.
Since the charge must be gas permeable, with prior art systems it is almost inevitable that the charge height, i.e. the layer of the particulate burden, must not exceed 500 mm and generally must be less than 500 mm.
With such low-height charges or burdens, however, full drying of the charge to the extent that it might be moist, in many cases is not achieved, but even more significant is the fact that with such low-height charges, the burden content of the product is markedly reduced.
It is true that using this carbothermic technique one can obtain ferroboron alloys or ferroborosilicon alloys that are practically free from detrimental contents of aluminum, the aluminum content being held generally to a maximum of 0.07% by weight. However, as noted, the boron content is then generally too low and the yield or recovery is unsatisfactory. For instance in the production of a ferroboron alloy, the boron content will generally be about 10% by weight. In the production of a ferroboron alloy the boron content is generally reduced to about 3% by weight for a silicon content of 3% by weight.
Efforts have been made to overcome this drawback by agglomerating the burden mixture or charge into comparatively large-size pellets so that within the furnace chamber a greater thickness of the burden comprised of the pelletized charge can be used. This, however, has also proved to be unsuccessful, and it can be said that the results obtained are basically no different even though one would expect the porosity of the burden to be increased.